Three options, with one for about $2,000.
#1. The Niles SI-1230 is a 12-channel power amplifier with 30 watts per channel (40 watts into 4 ohms), and it retails for $1,350.
Or the Niles SI-1260 has 12 channels rated at 60 watts per channel (80 watts into 4 ohms), it retails for $2,250.
* Now you need an additional 8-channel amp to power all your 20 speakers.
The Rotel RMB-1048 is your ticket, with 8 power amps rated at 40 watts per channel (60 watts into 4 ohms), and it retails for only $799.
#2. [Best option] Or get two of them (Rotel), and add a 4-channel amp, like the NAD C 945BEE (40 watts per channel/8 ohms), retails for only $499. Use the NAD for the zone with 4 speakers.
Total cost, less than $2,100. This is your best value.
And I'm sure that you'll get a good discount too.
#3. As an alternative for a 12-channel power amp, there is the Russound R1250MC, that retails for only $1,499 (50 watts into 8 ohms, and 80 watts into 4 ohms per channel).
>>> So, if you have a party, and like to have music in all your zones, you're in business.

The #2 option is the best one, and if I was in the same situation than yours, that will be the one that I personally choose for my home.
Bob